Stimulation of the growth of muscle cells in culture by hormones with well-established anabolic effects in whole animals is being investigated. The hormones under study are growth hormone, insulin, and the somatomedin analog, MSA. We place special emphasis on the relative activity of these hormones at or near the concentrations at which they are found in the blood of normal animals. We have found that the principal stimulator of myoblast proliferation in culture is the somatomadin analog, and that this effect is paralleled by actions of purified human Somatomedin A. We are currently studying actions of these growth factors and their interactions with muscle cells in culture.